Photochemistry volume 40

Photochemistry
Volume 40



A Specialist Periodical Report

Photochemistry
Volume 40
A Review of the Literature Published between
May 2011 and April 2012
Editor
Angelo Albini, University of Pavia, Pavia, Italy

Authors
Serena Berardi, University of Padova, Italy
Marcella Bonchio, University of Padova, Italy
Sebastiano Campagna, Universita` di Messina, Italy
M. Consuelo Jime´nez, Universidad Polite´cnica de Valencia, Spain
Elisa Fasani, University of Pavia, Italy
Bernd Herzog, BASF Grenzach GmbH, Germany
Haruo Inoue, Tokyo Metropolitan University, Japan
Giuseppina La Ganga, Universita` di Messina, Italy
Roland Lindh, Uppsala University, Sweden
Ya-Jun Liu, Beijing Normal University, China
Ugo Mazzucato, Universita` di Perugia, Italy
Alberto Mezzetti, CNRS, France
Miguel A. Miranda, Universidad Polite´cnica de Valencia, Spain
Kazuhiko Mizuno, Nara, Japan
Stefano Protti, University of Pavia, Italy

Fausto Puntoriero, Universita` di Messina, Italy
Daniel Roca-Sanjua´n, Uppsala University, Sweden
Andrea Sartorel, University of Padova, Italy
Takashi Tsuno, Nihon University, Japan


If you buy this title on standing order, you will be given FREE access
to the chapters online. Please contact with proof of
purchase to arrange access to be set up.
Thank you.

ISBN: 978-1-84973-437-0
ISSN: 0556-3860
DOI: 10.1039/9781849734882
A catalogue record for this book is available from the British Library
& The Royal Society of Chemistry 2012
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For further information see our web site at www.rsc.org


Preface
DOI: 10.1039/9781849734882-FP005

Volume 40 completes the new course of the periodic reports on photochemistry, where the topics are reviewed every other year. Thus, the physicochemical and inorganic aspects as well as solar energy conversion have
been reviewed in Volume 39, while the present one includes the organic
aspects and computational photochemistry. A general introduction and
review referred to both years 2010 and 2011 is included in the present
volume. As discussed previously, this structure seems more appropriate to
the needs of present day research, where having an organized summary of
the work going on in the various areas of photochemistry is more important
than receiving an ultrafast information about a new paper. Everybody has
now a rapid access to the literature through instruments different from a
yearly book, and in that capacity the present series would certainly be a
poor competitor.
The organic aspects are presented in four chapters, as in previous
volumes, and one chapter is devoted to the core physical and computational
aspects. We welcome here Professor Liu as a new contributor.
Adding to the review chapters a series of highlights devoted to important
aspects of applied photochemistry has become an established feature of the
series. In the present volume, five highlights are presented. These involve
one of the industrially most significant, one may say ‘mature’ applications,
UV filters, and two typical aspects of ‘academic’ (at present, but preparsing
future applications) topics, such as light-induced water oxidation and the
complex equilibria of flavanols. Then, two chapters concern the history of

two of the main Photochemical Societies worldwide, the European Photochemistry Association and the Asian Oceanis Photochemistry Association.
Photochemistry as a science owns much to the effort for cultural communication and development national and international associations did,
particularly in the Sixties. Changes, mergings, developments have then
taken place and present day associations are quite differente and serve a
different function. It seemed timely to present these historic contribution,
further additions will follow.
I regret the untimely loss of Professor Luis Serrano-Andre´s, who contributed to this series. Finally, I thank the staff of Specialist Periodical
Reports and my colleagues of the Photochemical Group at the University
of Pavia for their help.
Angelo Albini

Photochemistry, 2012, 40, v–v | v

c

The Royal Society of Chemistry 2012



CONTENTS
Cover

In 1912 Giacomo Luigi Ciamician, ‘‘The
Father of Photochemistry’’ opened his
address to the International Congress of
Applied Chemistry with the paragraph
detailed on the cover. Photochemistry of
the Future has been an inspiration to the
field of Photochemistry ever since.

Preface
Angelo Albini

v

Periodical reports: Organic and computational aspects
Introduction and review of the years 2010–2011
Angelo Albini
1 Introduction
2 Review of the years 2010–2011
References

3
5
36

Computational Photochemistry and Photophysics: the state of the art

42

Ya-Jun
1
2
3
4
5

42
43
44

48
49

Liu, Daniel Roca-Sanjua´n and Roland Lindh
Introduction
The most basic concepts in photochemistry
A brief history of computational photochemistry
A critical point of view on methodology
Development of computational photochemistry
2010–2011
6 Conclusion and outlook
Acknowledgments
References

3

66
67
67

Photochemistry, 2012, 40, vii–x | vii

c

The Royal Society of Chemistry 2012


Alkenes, alkynes, dienes, polyenes

73

Takashi Tsuno
1 Photochemistry
2 Photochemistry
3 Photochemistry
4 Photochemistry
5 Photooxidation
References

73
94
96
96
98
99

of
of
of
of

alkenes
dienes
polyenes
alkynes

Photochemistry of aromatic compounds

106

Kazuhiko Mizuno
1 Introduction
2 Isomerization reactions
3 Addition and cycloaddition reactions
4 Substitution reactions
5 Intramolecular cyclization reactions
6 Inter- and intra-molecular dimerization reactions
7 Lateral-nuclear rearrangements
References

106
106
108
117
119
129
133
136

Organic aspects. Oxygen-containing functions

146

M. Consuelo Jime´nez and Miguel A. Miranda
1 Norrish type I reactions
2 Hydrogen abstraction
3 Paterno`-Bu¨chi photocycloadditions
4 Photoreactions of enones and quinones
5 Photoelimination
6 Photo-Fries and photo-Claisen rearrangements

7 Photocleavage of cyclic ethers
References

146
147
154
156
160
165
167
168

Functions containing a heteroatom different from oxygen

174

Angelo Albini and Elisa Fasani
1 Nitrogen containing functions
2 Functions containing different heteroatoms
References

174
186
189

viii | Photochemistry, 2012, 40, vii–x


Highlights in photochemistry
The history of the European Photochemistry Association

Ugo Mazzucato
1 Preliminary contacts for a new Association
2 The Foundation and first steps of the European
Photochemistry Association (1970–76)
3 The EPA in its mature period (1977–2000)
4 EPA in the last decade: a slackening period and a prompt
revival
Appendix - The history of the EPA Newsletter
Acknowledgements

197

History of the Asian and Oceanian Photochemistry Association (APA)
Haruo Inoue
1 Foundation of the Asian and Oceanian Photochemistry
Association (APA)
2 Birth of the APA
3 Pre-history of the APA
4 Activities of the APA and the regional societies in Asia and
Oceania
Appendix

230

Photoprotection of human skin

245

Bernd Herzog
1 Ambient UV radiation and properties of human skin

2 UV filters for sunscreens
3 Sunscreen formulations and their assessment
4 Understanding sunscreens
5 Conclusion
Acknowledgments
References

245
250
259
265
269
270
270

Photo-induced water oxidation: New photocatalytic processes and
materials
Serena Berardi, Giuseppina La Ganga, Fausto Puntoriero,
Andrea Sartorel, Sebastiano Campagna and Marcella Bonchio
1 Introduction
2 Photo-induced water oxidation
3 Photosensitizers for water oxidation

197
199
206
220
226
229

230
235
236
237
243

274

274
275
277

Photochemistry, 2012, 40, vii–x | ix


4 Oxygen Evolving Catalysts
5 Towards the device: Anchoring the catalysts onto electrodes
6 Conclusions and Outlook: the artificial leaf
Acknowledgements
References

280
286
287
290
291

Any colour you like. Excited state and ground state proton transfer in
flavonols and applications

295

Stefano Protti and Alberto Mezzetti
1 Introduction
2 3-Hydroxyflavone (3HF) as a model molecules for proton
transfer processes
3 Interaction of 3HF and natural flavonols with biomolecules
4 Photophysical behavior of synthetic 3-hydroxyflavones
and their use as fluorescent probes
5 Technological applications of flavonols
6 Synthetic applications of flavonols
Acknowledgments
References

x | Photochemistry, 2012, 40, vii–x

295
296
300
302
310
314
316
316


Periodical reports: Organic and
computational aspects

Introduction and review of the years
2010–2011
Angelo Albini
DOI: 10.1039/9781849734882-00001

After a short introduction on the changes adopted in the format of this series, some
representative findings on photochemistry and applications published in 2010–11 are
reviewed.

1

Introduction

The present volume, no. 40 in the series ‘Photochemistry’ of the Specialist
Reports published by the Royal Society of Chemistry makes a further step
forward in the direction indicated in volumes 37–39. This choice arises from
the idea that the role of photochemistry has changed by a large degree in the
more than 40 years intervening since then the series was planned (volume 1
was published in 1970). In the Sixties, photochemistry was a young science
(see below, however) that had been just established as a consistent discipline
and the advancement in the rationalization of key issues was pointed out
year after year by each volume. This fact, along with the much greater work
then required for literature search, made these series a much wellcome
opportunity for the many scientists then entering the field and for anybody
wishing to keep abreast with the advancement of this discipline in a timeeffective way. Nowadays, literature search is done in a much faster,
although not necessarily dependable, way, while photochemistry has
become a pervasive science with a variety of remarkably diverse
applications.
Thus, the problem is not so much that of making available new notions

to the photochemical comunity, but rather that of offering the information
to various communities of scientists, some of which do not consider themselves full-time photochemists, and facilitate the exchange between them.
Indeed, differently for example from some spectroscopic methods, where
having a crytically compiled list of the data published each year remains
useful, offering inventories of the new publications in photochemistry is
probably not sufficient. Thus, after that with the previous two volumes
the delay accumulated had been eliminated, it was felt that a structure
change was advisable. Thus, next (yearly) volumes will be prepared in the
following way.
The periodical on the different photochemical disciplines will be published every other year. The biennal coverage should help in clarifying the
development of specific studies and their significance for photochemistry in
general, while it is hoped that the delay in reporting part of the data has a
limited effect, because appropriate literature surveys are generally available.
Of course, the short review of the last two years that is done in this chapter
Dipartimento di Chimica, Universita` degli Studi di Pavia, Via Taramelli 12, 27100 Pavia,
Italy. E-mail:

Photochemistry, 2012, 40, 1–41 | 3


refers to the whole field of photochemistry, indeed is meant to give a flavour
of the large field of applications.
The specific reports mentioned above will correspond to about a half of
each volume, the other half being occupied by highlights, prepared by well
known specialists. For the reasons mentioned above, these will be mainly
devoted to applicative aspects of photochemistry.
It is hoped that this dual structure may contribute to maintaining some
connection among the various fields of photochemistry, whether these refer
to the core discipline or to a practical application.
As a result, volume 39 contains reports on spectroscopic and physicochemical aspects (coverage: year 2010), as well on inorganic aspects and

solar energy conversion (among the Authors, F. Punturiero and K.
Kalyanasundaran contribute for the first time to this series), while organic
and theoretical aspects are reviewed in volume 40 (two years coverage, 2010
and 2011, Y. Liu contributing for the first time).
As for the highlights, these had been introduced in volumes 37 and 38 in
the number of three and five respectively and should remain in that range, as
it is the case for the present volume. Next to scientific reports, a historic
account on two of the main photochemical societies, the Asian and Oceanian and the European, are presented.
Two further topics should be rapidly mentioned. The first one as to do
with history. In July 2010 a minisymposium for celebrating the 100th
birthday of photochemistry was organized. The choice of the date may be
discussed. This originated from the recognizment that, although the action
of solar light on a variety of chemicals had been long known and some
photochemical reactions had been well described in the 19th century and
earlier, it is only through the work by Giacomo Ciamician, Emanuele
Paterno`, Hans Stobbe and a few others that a sufficient number of reactions
was thoroughly studied, so that generalizations could be made.1 The work
by these scientists was for the main part published by 1910 and by that year
many – if not most of the – photochemical processes that today are applied
in the lab and taught in the classes were known.
Apart from some historic note, the meeting attempted to re-create the
spirit of a hundred years ago, when photochemistry seemed to be the science
of the future. This was done through seven lectures figuring out what may
be the contribution of photochemistry to the development of chemistry
(solar light conversion, organic synthesis,2 molecular machines,3 single
crystal photochromism,4 computation and photochemistry,5 new chemistry
and biology via singlet oxygen,6 photomedicine),7 as well as by asking every
participant which he/she felt the most important contribution photochemistry may give in the future.8
Finally, one may ask the question, which is the place of photochemistry at
present? Perhaps not the science of the future, as it was in the first decade of

the 20th century, nor it is expanding as it did in the 1950 and 1960. Certainly,
it pervades chemistry, physics, biology and allows advancement that would
not be possible withouth the insight we now have of photochemical
processes.
One way for assessing how important is deemed this discipline is looking
for the most often red papers. As an example, the American Chemical
4 | Photochemistry, 2012, 40, 1–41


Society publishes a list of the ten most accessed papers for each of its
journals in 2010. It is remarkable that six out of the ten most accessed
articles in Accounts of Chemical Research is directly concerned with photochemistry (this is in part due to the printing of a special issue on the
theme, but this is not sufficient to explain the great success of the topic). As
it appears from the titles below, five of these have to do with the various
aspects of solar energy conversion. It would appear that investigations on
this problem are again experiencing a lively development.
– Recent Advances in Sensitized Mesoscopic Solar Cells.9
– ‘‘Plastic’’ Solar Cells: Self-Assembly of Bulk Heterojunction Nanomaterials by Spontaneous Phase Separation.10
– Molecular Understanding of Organic Solar Cells: The Challenges.11
– Solar Fuels via Artificial Photosynthesis.12
– Visible Light Water Splitting Using Dye-Sensitized Oxide
Semiconductors.13
– Using Singlet Oxygen to Synthesize Polyoxygenated Natural Products
from Furans.14
This impression is confirmed by the fact that the other ACS journal where
photochemical articles are among the top ten is Inorganic Chemistry with
four.
–
–
–

–

Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells.15
Chemistry of Personalized Solar Energy.16
Catalytic Water Oxidation by Single-Site Ruthenium Catalysts.17
Photoelectrochemical Behavior of Sensitized TiO2 Photoanodes in an
Aqueous Environment: Application to Hydrogen Production.18

However, organic chemistry and material science are not cut down. An
indication is the presence in the list of an account on the synthetic utility of
singlet oxygen (see above)14 and of a paper on click chemistry for surface
immobilization in the specific JACS list for surface patterning.
– High Density Orthogonal Surface Immobilization via Photoactivated
Copper-Free Click Chemistry.19
2

Review of the years 2010–2011

2.1 Books, reviews
After the two textbooks published in 2009, two further photochemical
books of general interest became available in 2010. One of them is a
handbook of synthetic photochemistry, addressing the practical issues of
how to carry out a photochemical preparation and reviewing in ten chapters
the main photoreactions.20 These are classed according to the chemical
transformation occurring (type of bond formed, linear or cyclic product
etc.) in order to facilitate the inclusion of photochemical steps in synthetic
planning. The second one is a two volume set (40 chapters, 1200 pages) on
hydrogen transfer in excited states.21 Then, in 2011 Ramamurthy and Inoue
edited a conspicuous (640 pages) book devoted to supramolecular photochemistry22 and Wypych published a Handbook of UV degradation and
stabilization.23 Further major multi-authors books or special issues in

Photochemistry, 2012, 40, 1–41 | 5


scientific journals has concerned photochromism,24 solar chemistry and
photocatalysis,25 the plenary lectures at the XXIII IUPAC Symposium in
Photochemistry.26
A number of excellent reviews have been published in various journals.
Besides those mentioned above and some more that will be indicated when
discussing the specific reactions below in this section, a few of the topics
considered are listed below. This serves at least to have a taste of how varied
is the scope of the applications of photochemistry.
– Using perfluoroazides for the modification of surfaces and the synthesis of nanomaterials.27
– Advances in patterning materials for 193 nm immersion lithography.28
– Fluorescent analogs of biomolecular building blocks: design, properties, and applications.29
– Imaging and photodynamic therapy: mechanisms, monitoring, and
optimization.30
– Beyond photovoltaics: semiconductor nanoarchitectures for liquidjunction solar cells.31
– Engineering Metal Organic Frameworks for Heterogeneous
Catalysis.32
– Recent Studies of Laser Science in Paintings.33
– Conservation and Research Role of the ps* State in Molecular
Photophysics.34
– Ultrafast Interfacial Proton-Coupled Electron Transfer.35
– Reviews photoinitiated polymerization: advances, challenges, and
opportunities.36
2.2 Organic synthesis
A special mention deserves the overview Nick Turro has published of his
work in physical organic, organic supramolecular and spin chemistry during
his five decades carrier at Columbia.37 A tutorial review has been published
on the utility of photolabile protecting groups in chemical synthesis and in

biology.38 A wide scope crytical review has been devoted to the 2 þ 2
cycloaddition reaction involving allenes and includes several photochemical
examples.39
A preparatively interesting synthesis of some Z-cynnamic acid derivatives
has been reported, based on the fact that the salts of these acids with amines
crystallize out of an acetonitrile solution (Scheme 1).40
The first examples of enyne [4 þ 4] adducts have been isolated from the
photocycloaddition to a 2-pyridone as a mixture of regio and stereochemical isomers. These are too strained to allow isolation, but the products
of further 2 þ 2 dimerizations have been identified. Further products formed
R

CO2H

R'
R''

hν

CO2H

R'
R''

Scheme 1

6 | Photochemistry, 2012, 40, 1–41

R

arise from 2 þ 2 cycloaddition (or from Cope rearrangement of the above
4 þ 4 adduct, see Scheme 2).41
2-Napthoquinone-3-methides are conveniently generated via the efficient
photodehydration (F = 0.2) of 3-(hydroxymethyl)-2-naphthols. These intermediates undergo facile hetero-Diels-Alder addition (kE4 Â 104 MÀ1 sÀ1) to
electron-rich olefins in an aqueous solution. In this way, photostable benzo[g]chromans are formed in excellent yield. The fraction of the quinone
methide that is not trapped is rapidly hydrated (k E 125 sÀ1) and regenerates
the starting naphthol. The fact that hydration competes with cycloaddition
makes the former process selective; actually only vinyl ethers and enamines are
sufficiently nucleophilic for adding.42 The photochemistry of 1,2-bis(butadienyl)benzene is affected by the introduction of methyl groups on the chain.
These limit planarity and thus alter absorption and photophysical parameters
and affect the competition between di-p-methane and 6 p e cyclization.43
Photoremovable protecting groups are becoming important in organic
synthesis and this increases the interest in the mechanism of fragmentation.
A study of two phenacyl phosphates showed that the reactive triplet has a
mixed np*/pp* character and its chemistry depends on the structure
(diphenyl or diethyl phosphate) and solvation (in accord with the prediction
from DFT calculations). Thus, in MeCN the triplet is long-lived (100 ns)
and essentially unreactive, while in more solvating media, such as fluorinated alcohols or mixed aqueous solvents, the triplet lifetime is shortened
to ca. 5 ns and rearrangement and cleavage occur (at least with a
good nucleofugal group, e.g. diphenyl rather than diethyl phosphate).
Hydrogen abstraction competes when the above conditions are not met
(see Scheme 3).44

R
C

R

C

+
N

O +

C

O

+
N

N

O

N

O

Scheme 2

O
OPO(OR)2
hν

100 fs

1MP(ππ*)

2–3 ns
3MP(nπ*/ππ*)

1MP(nπ*)

MeO
MP, R=Ph, Et

R'OH

O

OR'
MeO

MeO

O

Scheme 3

Photochemistry, 2012, 40, 1–41 | 7


A related investigation on 2-methylphenacyl epoxides has been carried
out for exploring the viability of preparing pharmaceutically active
hydroxyalkylindanones via hydrogen abstraction from the methyl group.
Some positive results were obtained, although the situation was complex, in
particular because of different cleavages competitively occurring, e.g. that of
the epoxide ring (see Scheme 4).45

A stereoselective Wolff rearrangement of a-diazo-N-methoxy-N-methylb-ketoamides (formed from enantiopure aminoacids) leads to enantiopure
b–lactams (see Scheme 5). The reaction is conveniently carried out in a
continuous-flow photochemical reactor made from inexpensive laboratory
equipment and is amenable to scale-up.46
Chiral photochemistry is an intrinsically difficult task, because this
implies controlling the shortlived, weakly interacting and highly reactive
species such as an electronically excited state. A welcome example, that
has been tagged ‘‘dual-chiral, dual-supramolecular’’ photochirogenesis
approach, has been applied to the [4 þ 4] photocyclodimerization of 2anthracenecarboxylate tethered to an R-cyclodextrin scaffold. The reaction
was accelerated by a g-cyclodextrin or cucurbit[8]uril host and gave a single
enantiomeric cyclodimer (out of four) in up to 98% chemical and 99%
optical yield.47
Higher members of the acene series have demonstrated to be a very difficult synthetic target. However, photochemical reactions can be carried out
at a low temperature and are thus well suited for arriving to products of
limited stability. A remarkable success has been the synthesis of octacene
OH
.

O

OH
.

O

OH
O

O

O

ISC
triplet
O

O.
.

O.

O

OH

O.
ISC
Scheme 4

Me

N

OMe

O

O

MeO

N2

X

O
N

NTr
O

NHTr

+

Me

NTr
O

X=OBn, CO2Me
Scheme 5

8 | Photochemistry, 2012, 40, 1–41

X

MeO

N
Me

O

X


and nonacene. Thus, doubly bridged derivatives prepared by Diels Alder
and elimination reactions have been oxidized to a-diketo derivatives. These
compounds have been irradiated at 30K, where long-wavelength irradiation
causes partial decarbonylation, but prolonged irradiation in the UV (several
hours) eliminated also the second bridge (see Scheme 6). High acenes have
been predicted to have antiferromagnetic properties and thus organic
materials of interest as semiconductors may be prepared by this path,
provided that one can devise a pattern of substituents that impart a sufficient kinetic stability to these compounds.48
The intramolecular Paterno`-Buchi reaction is a way for generating highly
strained compounds. As an example, the Diels-Alder adducts between 2cyclohexenone and 2-cycloheptenone were found to give strained polycyclic
oxetanes upon irradiation (see Scheme 7). The products suffered smooth

n
oxidation
O

O

O

O

n

hν, >360 nm
O

O

n
hν, 305–320 nm

n
n = 0, 1
Scheme 6

O

OH
+

hν
O

HCl

Cl

O
Scheme 7

Photochemistry, 2012, 40, 1–41 | 9

ring cleavage and subsequent carbocationic rearrangement under acidic
conditions giving highly functionalized compounds. Strikingly, even the bisadduct of cyclohexadiene and benzoquinone was found to be photoactive,
producing a C22-symmetric dioxetane via the monooxetane.49
As seen above, photochemical reactions are particularly useful for producing strained compounds. Another possibility these offer is forming
reactive intermediate under mild and versatile conditions, as it is the case for
radicals. The generation of radicals via oxidation-deprotonation of enols
induced by a photoexcited aromatic nitriles has been well characterized and
is finding new applications.
As an example, this apply to enols or tautomeric enols such as maleic acid
derivatives. While with a chemical reagent (cerium ammonium nitrate) the
only process occurring is oxidative dimerization, when aromatic nitriles are
used as the photochemical oxidant, selective trapping of the radicals by an
electrophilic alkenes or by the nitrile itself occurs. Under these conditions,
both the alkylation of alkenes and the oxidative alkylation/dimerization of
dienes have been smoothly obtained (see Scheme 8) and side processes such
as double alkylation or polymerization often occurring with other methods
have been avoided. A three-component (Nucleophile-Olefin Combination,
Aromatic Substitution) process is also possible.50
Radical can be smoothly generated also via a reductive path. Thus, a
household light bulb is sufficient for obtaining radical cyclization reactions
when a photoredox catalyst absorbing in the visible, such as tris(2,2 0 bipyridyl)ruthenium dichloride, is used. The single electron reducing agent,
Ru(I), is produced and activates a C-Br bond.51 The resulting radical
attacks indoles and pyrroles (see Scheme 9).
The energy of excited states can be used for the controlled/extensive
degradation of a variety of compounds. Effective hydrogen abstractors such
as quinones can be used for inducing DNA cleavage52 or for complete
degradation, interesting when selective for a class of compounds. A recent
report presents the cleavage of oligosaccharides by photoirradiation using
AQ/boronic acid hybrids under neutral (and exceptionally mild) conditions.
The anomeric hydrogen is selectively abstracted by the appended anthraquinone and an hexose unit (see Scheme 10).53

R
EWG
EWG

Sens,
Base
–e–,

–H+

EWG

R''

.

R'
R

R'''

EWG

R''

R'

EWG

R'''

EWG

EWG
EWG
EWG = Electron Withdrawing Group
Scheme 8

10 | Photochemistry, 2012, 40, 1–41

2


EtN3
RuII(bipy)3*

Et3N+ .

hν
RuII(bipy)3

RuI(bipy)3*

N

N
Br

CO2Me

CO2Me

.

H

N
H

CO2Me
CO2Me

CO2Me
CO2Me

Scheme 9

2.3 Mechanism, intermediates
An issue of general interest is the comparison of the processes in ultrafast
time-resolved mass spectrometry with the ‘‘normal’’ photochemistry on
lower electronic surfaces occurring in solution. The dynamics of high-lying
states formed by excitation proceed along steep potential energy surfaces and
conical intersections. The dynamics are here much faster than vibrational
relaxation, and the course of the reaction depends on the energy initially
supplied. Typical examples of such unusual processes taking place upon
excitation at 6 eV, are trans-stilbene that undergoes a phenyl twisting motion,
cis-stilbene that suffers an ultrafast cyclization to dihydrophenanthrene and
azobenzene that reacts via an ultrafast fragmentation.54
Another concept of general interest that has been revised is the use of the
Rehm-Weller equation for predicting the rate of electron transfer processes

involving excited states. A recent investigation found significant discrepancies in both DG and kq values obtained in this way. Rather, the
revised data were in good accord with the Sandros-Boltzmann equation
kq ¼ klim =½1 þ exp½ðDG þ sÞ=RTŠŠ
a fact that appears to reflect the rapid interconversion among the encounter
pairs and the exciplex

Á

ðA*=D ! exciplex ! A À=D

Á þÞ:

In the equation above, the quantity klim is a measure of the diffusionlimited rate constant, and s of the free energy difference between the radical
ion encounter pair (Ad À /Dd þ ) and the free radical ions (Ad À þ Dd þ ). The
quantity s is positive,=0.06 eV, because the former pair is less well solvated,
Photochemistry, 2012, 40, 1–41 | 11


H
H
O
HO - O
B

H

OH

H
HO

N+

H

H

H

H
HO

H

O

O

O HO

O

H
OH

O

O

hν
H

H

O
O
HO - O
B

O

O
H
H
HO

N+

O HO
OH .
H
H O

O

O
H

H

H
HO

H
OH

.

O
O

O

O2
H
H
HOHO

O HO

O
H
H
HO

OH
OO
H

.

O

O
H
H
HO

H
H
OH
H
HO
HO
H
HOHO

+

O
H
H
HO

OH

O

O

O
H

H
HO

H
H
OH

H

Scheme 10

whereas the correction to DG assumed in the Rehm Weller equation is
negative (=À0.06 eV).55
The vexed question of hydrogen transfer vs electron transfer mechanism
in the photoreactions of aromatic ketones has been confronted in a laser
flash photolysis study in the presence of a good donor such as 2-aminobenzimidazole. Benzophenone (np* triplet) and 2-benzoylthiophene (pp*
triplet) showed a quenching rate constant quite similar (6.2 and 3.9 Â 109
MÀ1 sÀ1, respectively). This suggested that the process is not a pure
hydrogen abstraction but rather a charge transfer followed by proton
transfer, in agreement with thermodynamic predictions. The mechanism
of proton-coupled electron transfer from tyrosine in enzymes is another
debated issue. A recent study on the intramolecular oxidation kinetics of
tryptophan derivatives linked to [Ru(bpy)3]2 units with water as proton
acceptor supplied evidence of two mechanism for oxidation.56
With 4-methoxy- and 4, 4 0 -dimethoxybenzophenones, two distinct
absorption bands corresponding to both types of triplets (np* and pp*)
were observed, both of which were quenched by the aminobenzimidazole
with a high rate (109 to 1010 MÀ1sÀ1). With a np* triplet, electron withdrawing-substituted ketone such as 4-carboxybenzophenone, the quenching
12 | Photochemistry, 2012, 40, 1–41

rate constant was higher (8 Â 109 MÀ1 sÀ1), close to diffusion control.
Density functional theory studies suggested the formation of ground state
complexes where excitation led to radical ion pairs.57
A study on nanocrystalline suspensions of substituted benzophenone
evidenced that the triplet lifetime in crystals span over nine orders of
magnitude from o0.1 ns to 1 ms, whereas in solution the corresponding
span is of three orders of magnitude (from 1 ms to 1 ms, electron-rich
derivatives are most efficiently deactivated).58 Exploring how photophysical
parameters change in the crystal state is informative about the mechanism
involved, in this case the mode of deactivation, but has interest also for
applications, such as organic Light Emitting Diodes or solar cells.
Multiphotonic processes under a high flux have not been often studied in
detail. A peculiar result has been found in a study of the photoreactivity
in solution of some aromatic dimers when exposed to nonresonant, intense
femtosecond laser pulses.
Thus, the cycloreversion of a biplanemer appeared to depend linearly on
the laser intensity, whereas the formation of anthracene from its photodimer was proportional to the cubic of laser intensity (see Scheme 11a). The
unusual result with the biplanemer was explained as combination of a threephoton intramolecular cycloreversion and the conversion back to the
reactant by a two-photon intramolecular cyclodimerization. The fact that
multiphotonic processes of different order could occur within the same laser
pulse was justified by the inhomogeneous spatial distribution of the laser
intensity. At the center of the laser focus, the intensity is higher and a three
photon process is favored, at the wings of the laser focus it is the twophoton process that predominates.59
Fragmentation processes have in several cases be documented through
the spectral and kinetic characterization of the intermediates. Radicals have
been detected upon flash photolysis of nanocrystals of a para-methoxy
substituted dicumylketone analogue suspended in water, a rare event at
room temperature (see Scheme 11b). This has been revealed by timeresolved electron paramagnetic resonance (TREPR). Under an externally
NCOCF3

NCOCF3
hν

F3COCN

F3COCN

hν

Scheme 11a

Photochemistry, 2012, 40, 1–41 | 13


applied magnetic field ISC populates the three triplet levels unevenly and
leads to a nonequilibrium spin state population.60
The exact mechanism of the Wolff rearrangement, in particular whether it
involves discrete intermediates, has been a matter of discussion. Ultrafast
time-resolved spectroscopy with UV/vis detection allowed the observation of
singlet benzoylphenylcarbene (absorption at 740 nm, decaying with a 150 ps
time-constant in acetonitrile) from the photolysis of azobenzil. On the other
hand, IR detection revealed the ketene (Wolff rearrangement, n 2100 cmÀ1)
and showed that it is formed by two parallel pathways, viz directly from the
diazo excited state (‘immediate’) and via the carbene with a ‘slow’ rise timeconstant of 660 ps (see Scheme 12). Photolysis of diazoacetone in chloroform
led mainly to the ketene through a concerted process.61
In a similar way, it has been debated whether unsaturated esters are
formed from diazoesters via the intermediacy of carbenes or directly by
rearragement in the excited state (see Scheme 13). Evidence in favor of the
latter mechanism has been obtained from a time-resolved IR study.62

Compounds containing a chain of five unsubstituted carbons (R-C5-R 0 )
are best envisioned as ground-state triplet dialkynyl carbenes (see Scheme
14). The study of such species may provide information on electrical conductivity at the molecular scale as well as on the formation of highly
O

O

Ar
Me

Ar
Me
Me

.

Ar
Me

Me

Me

Ar
Me

.

Ar
Me

Me

.

CO

CO
.

Me

Ar
Me

Me

Ar
Me

Ar
Me
Me Me

Ar = 4-MeOC6H4
Scheme 11b

1

N2

R'

R
hν

R'

R'

R

C C O
O

O

R

N2
R'

R
O

products

R'

R

O

R, R' = H, Me; R = R' = Ph
Scheme 12

R
N2
R

OMe

O

hν

O

OMe

R

OMe
O

Scheme 13

14 | Photochemistry, 2012, 40, 1–41

products